1. Field of the Invention
The present invention relates to a molding method and, more particularly, to an improved molding method, especially adapted for splicing high voltage electrical cable in a reliable and efficient manner.
2. Description of the Prior Art
Relatively low voltage electrical cable generally consists of one or more centrally disposed electrically conductive cores or wires surrounded by a layer of electrically insulative material. Primarily, the insulative material is to protect against electrical shock and against short circuiting. At times, it becomes necessary to connect another segment of electrical cable to an existing cable; this is commonly referred to as a "splice." For low voltage cable, often splicing simply entails connecting the conductive cores together and covering the cores with insulative material, such as insulative tape.
With the ever increasing requirements for more power, higher and higher voltages are being transmitted along electrical cables; of these, underground cables require reliable insulation. For many years oil-filled cables fulfilled the insulation requirements. However, oil-filled cables are expensive to manufacture, to install and to maintain. In recent years, high voltage cables having solid dielectric materials have been developed. These materials include polyethylene, cross-linked polyethylene and ethylene propylene rubber (EPR). Using these materials, some cables have been designed to transmit voltages as high as 138 kilovolts. High voltage cables, however, present some difficult problems when compared to low voltage cables since there is a far greater propensity for failure due to a short circuit. Should there be an imperfection in the high voltage cable's conductive core, an excess charge will build up at the imperfection and create a corona discharge; that is, there will be a discharge through the insulative material into the surrounding environment. The imperfection is more generally referred to as a "discontinuity." For example, if a copper wire is used as the electrically conductive core, any burr or scratch on the wire surface will serve as a discontinuity and initiate a corona discharge. Such a discharge is usually fatal to the cable since there will be a progressively increasing amount of dielectric stress generated in the insulative material until failure occurs.
One method for solving the problem of corona discharge is to provide a layer of semi-conductive material about the conductive core. This semi-conductive layer may be partially of synthetic resin which will allow it to be molded about the conductive core so as to present a relatively smooth outer surface along which the high voltage charge will travel. Splicing high voltage cable, however, is very difficult since any discontinuity introduced by the splicing process may engender a corona discharge. One common problem is the introduction of entrapped air; the entrapped air forms a discontinuity and may cause a corona discharge. Two basic requirements for a reliable high voltge splice are first, that the added insulating material be compatible with the original insulating material and, second, that the added insulating material have a dielectric constant similar to the dielectric constant of the original insulating material. To be otherwise risks the creation of a discontinuity or "electrical interface" along which ionization may take place or water may enter. In the high voltage hand-taped splices, an attempt is made to minimize this effect by making long tapers, that is "penciling" the cable insulative material prior to splicing to increase the length of a potential ionization path.
In addition to the technical problems involved, any system developed to prevent discontinuities during splicing should be a relatively economical one. For example, excessively high labor costs, such as those required when a highly specialized talent is necessary to operate the system, or any system which is not adaptble to portable field usage will never meet the requirements deemed necessary.
A traditional method for splicing low voltage cables using a molding device required the spliced section to be covered with an excessive amount of electrically insulative unvulcanized material usually in tape form. Next, the mold would be placed around the spliced area and heat and pressure applied; however, because of over packing, the molding device could not be fully closed thereby allowing the unvulcanized material to flow outwardly between the mold halves. This movement from a high pressure region to a low pressure region created a tension force on the conductive core causing the core to move. A misalignment due to the movement presents little problem for low voltage cables; but such a misalignment could present major problems for high voltage cables by creating discontinuities, and in some cases prevent the conductive core from receiving a complete covering of insulative material. The molding device also tended to entrap air which is inevitably included with each turn of the unvulcanized insulative tape. There is no egress through which this entrapped air may escape. Once again, the effect on high voltage cables of entrapped air is far different than the effect on low voltage cables. Hence, such a molding device is not suitable for high voltage splicing.
Another splicing technique for low voltage cables consisted simply of hand taping insulative material about the conductive core. While results for low voltage cables were satisfactory, problems and expense increased as the technique was applied to higher voltage cables. First, it became necessary to make long "pencils" or tapers prior to splicing to increase the potential ionization path thereby lessening the chance of a path developing; second, the taping process was time consuming, laborious and gave unreliable results. A newer system developed for higher voltage cables comprised the use of a plastic mold placed about the spliced section to act as a container for a liquid insulative material which is then poured into the mold. Since neither sufficient heat nor pressure is used, bonding is totally unrealiable so that water leakage is enhanced, and the likelihood of air bubble discontinuities is great. As mentioned, water leakage or air bubbles will cause a short circuit and failure of the cable.
Still another method developed for high voltage cables comprised wrapping insulative tape about the slice, covering this tape with a mylar tape and surrounding the splice with steel bands. Next, heating blankets were applied in an attempt to fuse the insulative tape. Problems with this method included long time periods to accomplish, on the order of 12 hours, extreme expense, the inevitable inclusion of air bubbles and an inability to function with some types of insulative materials such as elastomers.